1. Field of the Invention
The present invention relates to a wireless phone that has both a sound and visual communication function based on a circuit connection using a cellular system, and a sound and visual communication function based on an IP connection using a local area communication system such as a wireless LAN.
2. Description of the Related Art
In a conventional wireless device capable of utilizing two different wireless communication systems, an inter-mode diversity scheme has been proposed as a method of using the two wireless communication systems together for improving sound quality. One inter-mode diversity method can be seen in Japanese Patent Publication No. 2002-111558 in which a diversity operation is performed for each frame or symbol in order to increase the inter-mode diversity effect.
However, such a method for improving sound quality is usable only in systems having the same wireless circuit characteristics, and cannot be used for a communication scheme that employs different types of switching systems, i.e., employs a system in circuit switching mode (i.e., a system based on circuit switching) and a system in packet communication mode (i.e., a system based on packet switching), in which transmission data formats are also different. In addition, the diversity operation is performed without taking into consideration the time difference of sound signals (or speech signals) between the two systems, a variation thereof, and sound characteristics, so it may fail to improve the sound quality.
In other words, the switching between a sound signal received and decoded through the circuit switching mode and a sound signal received and decoded through the packet communication mode is implemented mainly using a handover scheme in which their reception qualities are estimated so as to perform the switching between the two sound signals, respectively, obtained through the two different modes.
FIG. 3 shows an example of the conventional wireless phone using the handover scheme. As shown in this figure, after a received signal is converted into a baseband frequency at an RF section 21, a controller 24 switches a switch 34B at regular intervals so that the received signal is input to both a CDMA/BB (BaseBand) section 22 and a wireless LAN/BB section 23. The controller 24 receives call quality information output from both the CDMA/BB and wireless LAN/BB sections 22 and 23 to determine their call quality levels.
The controller 24 selects the signal with the higher of the two call quality levels, and thus switches the switches 34A and 34B so as to use a corresponding one of the CDMA/BB and wireless LAN/BB sections 22 and 23.
A CDMA sound codec is designed to secure a desired level of sound quality even under bad communication conditions such as variable bit rates. On the other hand, a wireless LAN system can satisfy a high SNR (Signal to Noise Ratio) requirement because it assumes a local area communication, and can simply secure high sound quality using a high bit rate codec.
However, even if the conventional system switches to one of the two modes having higher sound quality with reference to the sound quality information, the conventional system could fail to improve the overall sound quality due to the processing time difference between the two modes, etc., because the CDMA and wireless LAN sound codecs employ very different communication schemes. In addition, in the case where the wireless link is congested, fluctuations of the sound signal occur due to packet losses resulting from packet collisions or the delay variation of packet transmission.
A communication environment that absorbs the fluctuations exists, but the fluctuation absorption cannot be expected in an ad-hoc environment, so the conventional system may not be suitable for the sound signal communication even though it has a high SNR.
In other words, as shown in FIG. 3, the conventional handover method does not complement the two modes (i.e., the circuit switching and packet communication modes) with each other, but the controller 24 determines their communication qualities so as to merely use one of the two modes having higher communication quality. That is, it does not use the two modes simultaneously for improving the sound quality.
The communication quality of sound signals in the packet communication mode is distorted due to the deterioration of the BER (Bit Error Rate), and the conventional handover method also cannot estimate the fluctuations that are caused by delays occurring in handling the transmission and reception timing of wireless packets, as mentioned above. Thus, it cannot detect the switching timing with high accuracy, and cannot effectively perform the switching operation between the circuit connection and packet communication modes for improving sound quality.